Fine structure of the micropylar apparatus of the chum salmon egg, with a discussion of the mechanism for blocking polyspermy

Abstract

The micropylar apparatus of the eggs of the chum salmon, Oncorhynchus keta, was examined by scanning and transmission electron microscopy. The apparatus consists of a funnel‐shaped vestibule, from the bottom of which extends the micropylar canal, about 5 μm, in diameter. The canal traverses the entire zona pellucida interna of the egg envelope, about 30 μm in thickness. At its inner end, the diameter of the micropylar canal is the same as the width of the sperm head, about 2 μm. The lower portion of the canal is occupied by a conspicuous outgrowth of the ooplasmic surface, 3–10 μm in length.The first contact of the ooplasmic surface with a fertilizing spermatozoon occurs at the apex of the outgrowth in the micropylar canal. The surface of the outgrowth undergoes changes immediately after penetration by a fertilizing spermatozoon that prevent supernumerary spermatozoa from penetrating the ooplasm. As the formation of the perivitelline space takes place, the outgrowth withdraws from the micropylar canal. Supernumerary spermatozoa, however, cannot enter the perivitelline space and are eliminated from the canal lumen. In addition, the micropylar canal decreases in diameter at the region near its inner end and forms a mechanical barrier to the passage of supernumerary spermatozoa.